Optical media having a high data density are ideal for storing large amounts of information, such as reference data bases, newspapers and books, library flies, etc. Consequently, this makes them valuable and a potential target for counterfeiting. It is believed that in the future the counterfeiting of optical media will become a problem. Currently, optical media are protected in a limited way by software or by electronic devices. In principle, security devices such as optical thin films, holograms, gratings and micro-prisms can be attached to optical media to protect them. However, in practice these attached security devices will to some extent affect the normal operation of the optical media or limit the working area. Therefore, it would be desirable for the media to have a built-in security feature which will not interfere with the normal operation of the media.
In addition to the counterfeiting of optical media, the counterfeiting of currencies, passports, credit cards, bank cards and other articles is also a concern. Currently, optically-variable security devices, such as thin films, holograms, gratings, micro-prisms, are commonly used to protect those articles; their optical features can be easily recognized by the public at large. Optically-variable thin film security devices exhibiting a color change with a shift in viewing angle have been disclosed in U.S. Pat. No. 3,858,977 issued in January of 1975 in the name of Baird et al. Such devices are successfully used on large denominations of Canadian bank notes and on drivers licenses in some jurisdictions. However it is believed that in the future, there will be a need for additional security. Also, in some applications, the ability to store digital data is required.
In an attempt to address the above issues, U.S. Pat. No. 5,009,486 issued Apr. 23, 1991 in the name of Dobrowolski et al. describes a form depicting, optical interference authenticating device. Dobrowolski et al. teach the introduction of additional patterns in thin film security devices by using masks during their deposition or by infrared laser burning to remove one or more layers of the thin film system. U.S. Pat. No. 4,838,648 in the name of Phillips et al. issued Jun. 13, 1989 discloses a thin film structure having both magnetic and optically-variable characteristics. The thin film structure includes a magnetic recording layer having encoded data, decodable by magnetic means.